There is a growing body of evidence that suggest that tumor associated macrophages (TAM) play a critical role in the progression of breast cancer from the benign to malignant state. In humans, epidemiological studies show that a high density of TAMs is associated with poor prognosis. In mice, ablation of macrophages inhibits the progression and metastasis of mammary tumors while over-supply accelerates these processes. The mechanisms by which TAMs exert their influence on tumor progression are as yet still ill defined. However, our recent studies in mouse models of breast cancer have shown that TAMs regulate the angiogenic switch as tumors progress to malignancy. This finding is consistent with clinical data that shows the association of TAMs with increased micro-vessel density in breast cancer. One possible mechanism to explain the TAM effect is through their production of Vascular Endothelial Growth Factor (VEGF) A as we have shown that they express this potent angiogenic factor. Recently, it has also been shown that macrophages, during development, express Wnt ligands and that these factors stimulate proliferation of vascular endothelial cells (VECs). TAMs also express Wnt ligands. Thus, this production of angiogenic regulatory factors by TAMs may explain their ability to potentiate angiogenesis and, thereby, enhance tumor progression. In this proposal we will assess the role of VEGFA and Wnt ligands produced by TAMs in tumor angiogenesis and progression, using sophisticated mouse genetics in a well-established mouse-model of breast cancer. The specific aims are: 1. To determine whether macrophage VEGFA is critical for tumor angiogenesis and whether TAM VEGFA is up-regulated by CSF1 in the tumor as it is in culture. 2. To determine whether TAM Wnts regulate tumor angiogenesis and progression. 3. To determine whether tumor VECs are TAM Wnt responsive and whether VEGFA regulates Wnt TAM expression. It is expected that the experiments proposed in the three specific aims will elucidate mechanisms by which TAMs enhance angiogenesis. Angiogenesis is a critical step in the establishment of tumors and a requirement for them to become malignant. Given the association of TAMs with microvessel density and poor prognosis in human breast cancers, the studies proposed should elucidate novel mechanisms in the regulation of this process that will have clinical relevance and suggest novel anti-angiogenic therapies. Project Narrative: Clinical and experimental evidence indicates that macrophages play a role in the progression of breast cancer. In part, this is through the regulation of blood vessel formation (angiogenesis), essential for tumor survival. This proposal will define the mechanism of macrophage action that should indicate novel therapeutic strategies.